1. Field of the Invention
The present invention relates to an image processing apparatus and, more specifically, to an image processing apparatus capable of eliminating moire patterns generated in reading dot images.
2. Description of the Prior Art
A digital copying machine has been known in which an original image is read by an image pickup device where the image is printed a paper by laser printer, and the like, based on the image data obtained therefrom.
However, there arises a problem in reading an original image such as a photograph or a painting having thick portions and thin portions provided as a dot image by a half tone etching method. More specifically, because of the relation between the pitch between each of the dots and the reading pitch by the image pickup device, or because of a subtle deviation of phase based on the periodicity in half tone processing, a periodical pattern of thick and thin portions called a moire pattern is generated, providing a causing trouble in viewing.
A method has been proposed to prevent the generation of the moire pattern, in which the dimensions or pattern of a Dither matrix is changed in half tone processing. However, the moire pattern can not be eliminated by this conventional method when the reading pitch (the pitch between pixels in the image pickup device) itself is the cause of the moire pattern. In view of the foregoing, the applicant of the present invention proposed, in the above mentioned related application, an image processing apparatus capable of eliminating moire pattern employing optical enlarging means for enlarging an original image to be read by an image pickup device and electrical reducing means for reducing the image by thinning out image data read by the image pickup device. Accordingly, by adjusting the rates of magnification of the optical enlarging means and the electrical reducing means in association with the dot pitch of the original, the moire pattern can be effectively eliminated. However, when a plurality of images having thick portions and thin portions (half tone images) having different pitches of dots are included in the original image, the moire pattern cannot be eliminated in all of the images.
In another method, the moire pattern is eliminated by arranging a filter for eliminating the moire pattern in a light path in reading the original image and by gradating the image by normally focused on one pixel of the image pickup device into adjacent pixels. However, the moire pattern cannot be eliminated in all of the images having thick portions and thin portions even by this method when a plurality of images with thick portions and thin portions having different dot pitches are included in the original image.